1. The Field of the Invention
The present invention relates generally to devices used to induce cardioplegia during surgical procedures on the heart. More particularly, the present invention is directed to a cannula used to administer cardioplegic solutions and other liquids into the heart in an antegrade direction with maximum effectiveness and minimum tissue damage.
2. Related Applications
U.S. patent application Ser. No. 187,230, filed Apr. 28, 1989, entitled Retrograde Venous Cardioplegia Catheters and Methods of Use and Manufacture naming Gerald D. Buckberg and Robert J. Todd as joint inventors and U.S. patent application Ser. No. 07/406,382, filed Sept. 12, 1989, entitled Cardioplegia Three-way Double Stopcock naming Robert J. Todd, Douglas L. Smith, and Michael N. Kelly as joint inventors, are now incorporated herein by reference in their entireties.
3. The Background Art
Since the early days of cardiac surgery, it has been recognized that in order to provide optimum surgical conditions when operating on the heart, it is necessary to reasons, an arrested, flaccid heart is preferred during a cardiac surgical procedure over a beating heart with blood flowing through it. Thus, in order to be able to efficiently perform cardiac surgery, it is often necessary to use cardiopulmonary-bypass techniques and to isolate the heart from its life-giving blood supply.
Research has shown that many deaths occurring after cardiac surgery were due to acute cardiac failure. At first, it was believed that the heart was simply beyond repair and that the operation had failed to correct the problem. Later, it was discovered that many of these postoperative deaths were due to new, and often extensive, perioperative (during or within 24 hours after the surgical procedure) myocardial necrosis (death of the heart tissue). Furthermore, many patients who survived were found to have suffered myocardial necrosis to a significant degree as a result of the surgical procedure, thereby resulting in low cardiac blood output.
It is now widely accepted that myocardial necrosis occurs because the energy supply or reserve of the cardiac muscle cells is inadequate to supply the needs of the heart during a surgical procedure where the normal operation of the heart has been interrupted. The availability of oxygen dramatically affects a cell's ability to satisfy these energy requirements.
For example, anaerobic metabolism of glucose produces two (2) moles of adenosine triphosphate ("ATP") per mole of glucose (as well as harmful acid metabolites), whereas aerobic metabolism of glucose produces thirty-six (36) moles of ATP per mole of glucose. This and other facts make it clear that continued cardiac activity during periods of decreased oxygen availability is extremely detrimental to the heart muscle. Therefore, one of the primary goals of myocardial preservation techniques during surgery is to reduce myocardial oxygen consumption.
Myocardial oxygen consumption is significantly reduced by stopping the electromechanical work of the heart. The oxygen demands of the beating empty heart at 37.degree. C. are four to five times those of the arrested heart (i.e., 4-5 ml/100-gm/min compared with 1 ml/100-gm/min). Buckberg, G. D., "Strategies and Logic of Cardioplegic Delivery to Prevent, Avoid, and Reverse Ischemic and Reperfusion Damage," 93 The Journal of Thoracic and Cardiovascular Surgery, 127, 136 (January 1987) (hereinafter referred to as: Buckberg, "Strategies and Logic of Cardioplegic Delivery").
Oxygen consumption can be reduced further by cooling the heart. For example, the oxygen requirements of the arrested heart at 20.degree. C. are 0.3 ml/100-gm/min and are reduced to only 0.15 ml/100-gm/min at 10.degree. C. On the other hand, the oxygen requirements of the beating or fibrillating heart at comparable temperatures, are 2-3 ml/100-gm/min. Buckberg, "Strategies and Logic of Cardioplegic Delivery" at 129.
The normal heart receives its blood supply through the left and right coronary arteries which branch directly from the aorta. Generally, the veins draining the heart flow into the coronary sinus which empties directly into the right atrium. A few veins, known as thebesian veins, open directly into the atria or ventricles of the heart.
A method which has been developed to preserve the health of the myocardium during surgery, and is now widely practiced, is the infusion of a cold cardioplegic fluid to cool the heart while at the same time stop the beating of the heart. After the initial infusion of cardioplegic solution to arrest the heart, the heart is again perfused with cold cardioplegic fluid approximately every thirty (30) minutes to maintain the cool, dormant state of the heart.
The use of cardioplegia to protect the myocardium has proven the most advantageous protective measure of those used to date. Cardioplegia, which literally means "heart stop," may be administered in an antegrade manner (through arteries, such as the aorta, in the normal direction of blood flow), in a retrograde manner (through veins, such as the coronary sinus, in opposition to the normal blood flow direction), or in a combination of retrograde and antegrade administration. Cardioplegic solutions, typically containing potassium, magnesium procaine, or a hypocalcemic solution, stop the heart by depolarizing cell membranes.
Cardioplegia may be induced immediately after extracorporeal circulation has begun, provided that the pulmonary artery is collapsed to attest to the adequacy of venous return. In antegrade cardioplegia, extracorporeal circulation is begun by inserting a single venous, return catheter into the right atrium to transfer blood from the body to the heart-lung machine which pumps the blood into the aorta above an aortic clamp.
After it is determined that venous return is adequate, a cannula is then inserted into the aorta beneath the clamp through which the cardioplegic solution is administered. The cardioplegic solution flows through the coronary arteries in the normal blood flow direction. As mentioned, cardioplegic solution is periodically administered to the heart throughout the surgical procedure.
When it is time to resume the normal function of the heart, one method to resuscitate the heart requires the use of warmed blood introduced into the heart through the antegrade cardioplegia cannula to reperfuse the heart. The use of warmed oxygenated blood as the reperfusion solution has been recognized as reducing damage to the heart. Buckberg, "Strategies and Logic of Cardioplegic Delivery" at 134. Thus, the antegrade cardioplegia cannula must be fabricated so that the blood cells do not experience damage (hemolysis) during their passage through the cannula. Cannulas having obstructions to fluid flow may cause hemolysis as the blood cells negotiate sharp corners and encounter turbulence as they pass through the fluid carrying lumen of the apparatus.
In view of the problems and needs present in the art, it would be an advance in the art to provide an apparatus for performing antegrade cardioplegia more efficiently and with less tissue damage than possible with previous apparatus. It would be another advance in the art to provide a cannula specifically adapted for performing antegrade cardioplegia which reduces the possibility of hemolysis during reperfusion. It would be a still further advance in the art to provide an antegrade cardioplegia cannula which is reliably and economically constructed.